Question

The Django docs recommend copying a model instance thus:

original.pk = None
original.save()

But if you “use inheritance” — apparently meaning if the model’s class is a subclass of a subclass of models.Model — you need to do it slightly differently.

Specifically, the doc says:

Due to how inheritance works, you have to set both pk and id to None:

and gives an example analogous to this:

original.pk = None
original.id = None
original.save()

This seems kludgey. In any case, I’d like to understand what’s going on. Why does using inheritance require you to set the id field to None also? Don’t all Django models inherit from models.Model in any case?

(NOTE: I’m omitting the bit from the doc about copying m2m fields, which incidentally seems even more kludgey.)

Answer 1

It’s because MTI (Multiple Table Inheritance), the type you’re talking about here, stores the object across multiple tables. Take this example:

class Animal(models.Model):
    ...

class Dog(Animal):
    ...

When you create a Dog, all the fields on Animal are saved into the table for Animal, and just the fields directly on Dog are saved to the table for Dog. When you lookup the Dog later, Django queries both tables and stitches them together.

Both tables, however need primary keys, and Django uses AutoFields for that, which are simply positive integer fields. So Dog has an id and Animal has an id. The pk is filled with the id for the Animal part because this is the main piece, and Dog‘s id doesn’t matter. However, if you’re going to make a copy, you need to copy both pieces. Otherwise, the Animal part will of the copy will not get it’s own Dog part of the copy.